ReviewAluminium carbide formation in interpenetrating graphite/aluminium composites
Introduction
Porous graphite preforms infiltrated with aluminium alloys are attractive materials for lightweight components such as internal combustion engine parts, due to the low thermal expansion, high thermal conductivity and self-lubricating properties of graphite. Such graphite/aluminium (C/Al) composites with an interpenetrating network structure can be produced by infiltration of a porous graphite preform with liquid aluminium alloys.
To achieve a specific mechanical or thermophysical property, the interface between graphite and aluminium is of great importance for the overall performance of composite materials. In particular, chemical reactions at the interface during processing or under service conditions can degrade the thermophysical properties of the composites. The main problems encountered in the development of C/Al composites are non-wetting conditions between the monolithic phases for short contact times, and the reactivity of graphite or carbon with liquid aluminium [1], [2], [3], [4], [5]. At 750 °C, the free energy of formation of Al4C34Al(l) + 3C(s) → Al4C3(s)is −168 kJ/mol [6]. The undesirable formation of aluminium carbides (Al4C3) at the C/Al interface is often observed in graphite/carbon fibre reinforced aluminium matrix composites infiltrated at high temperatures and low cooling rates. In this case, the formation of interfacial Al4C3 is accompanied by fibre degradation, and a deterioration of the mechanical properties of the composite is observed [2], [7], [8].
Aluminium carbide crystals are not only brittle, but also highly sensitive to moisture contact and thus promote accelerated fatigue crack growth rates due to their hydrophilic nature. The dissolution of aluminium carbide in water can be described as follows [9]:Al4C3(s) + 12H2O(l) → 4Al(OH)3(s) + 3CH4(g)
It is assumed that the heterogeneous nucleation of Al4C3 is associated with surface defects on carbon fibres, such as exposed edges of graphite basal planes that exhibit carbon atoms with uncompensated high-energy electron bonds [8], [10]. In addition, the degree of graphitisation of the fibres determines the fibre's reactivity to nucleation of Al4C3 [11]. Thus, fibres with a high degree of graphitisation (containing highly oriented basal planes with few exposed edges) and a smooth surface with little topography are ideal for hindering the nucleation of carbides, due to their lack of surface defects.
In order to suppress the formation of Al4C3 – independently of the level of graphitisation of the reinforcing components (particles, fibres or preforms) – carbon atoms must be prevented from dissolving and their movement across the interfacial boundary has to be avoided. One approach is to coat the reinforcing component with an inert layer, which acts as a diffusion barrier between carbon and aluminium [4], [12]. A second approach considered in this study, and one much more suitable for large-scale production, is the addition of an appropriate alloying element to pure aluminium to reduce the solubility of carbon atoms in aluminium. Si is known to reduce the tendency towards aluminium carbide formation in C/Al–Si composites [13].
In the present study we used the gas pressure infiltration (GPI) method to produce interpenetrating C/Al composites. The cooling rate from the infiltration temperature is much lower in the GPI method than in indirect squeeze-casting [14]. Consequently, the probability of aluminium carbide formation is enhanced. Here we investigated the influence of different amounts of Si on the formation of aluminium carbide by carrying out infiltration runs at 750 °C. The presence of aluminium carbides was verified by optical and scanning electron microscopy, X-ray diffraction (XRD), and by a chemical method. Finally, four-point bending tests were performed to investigate the influence of aluminium carbides on the mechanical properties in the interpenetrating C/Al composites.
Section snippets
Monolithic materials
For the infiltration runs we used commercially available graphite FU2590 with an open porosity of 10 vol.%, manufactured by Schunk Kohlenstofftechnik GmbH, Germany. The thermophysical properties of this graphite preform can be found in [15]. The porous performs, with a dimension of 150 mm × 75 mm × 14 mm, were infiltrated by the gas pressure infiltration method at the Leichtmetall-Kompetenzzentrum Ranshofen (LKR), Austria. Pure aluminium (99.8 wt.%) and various Al–Si alloys were infiltrated (AlSi7,
Microstructural characterization
Fig. 1 illustrates a typical microstructure of the interpenetrating C/Al composite in question. The bright phase corresponds to aluminium, whereas the dark grey phase represents graphite and the small black spots residual porosity. At higher magnifications (see Fig. 2a), composites infiltrated with pure Al reveal the presence of needle-like phases that have grown from the C/Al interface into the aluminium phase (marked by arrows). Fig. 2a shows the surface directly after polishing. To verify
Conclusion and summary
Gas pressure infiltration at 750 °C of various Al–Si alloys into porous graphite preforms leads in all composites to the formation of aluminium carbides, independent of the Si content. Although the amount of carbides decreases with increasing silicon content, even eutectic and hypereutectic specimens exhibit distinct carbide formation. Chemical degradation of those aluminium carbides exposed to moisture occurred within a few days. The most important parameter obviously influencing the growth
Acknowledgements
The authors would like to thank the European Commission for funding part of this work under Contract No. G3RD-CT-2002-00799. Help with SEM imaging and XRD measurements from M. Siegrist and B. Zberg is also gratefully acknowledged.
References (21)
- et al.
Mater. Sci. Eng. A
(2004) Mater. Sci. Eng. A
(1991)- et al.
Mater. Sci. Eng. A: Struct. Mater. Prop. Microstruct. Process.
(1999) - et al.
Scr. Mater.
(1997) - et al.
Carbon
(1994) - et al.
Mater. Sci. Eng. A
(2004) - et al.
Carbon
(2003) - et al.
Compos. Sci. Technol.
(1996) - et al.
Mater. Sci. Eng. A
(1995) Mater. Lett.
(1998)
Cited by (183)
Aluminum carbide formation in Al-graphite composites: In situ study and effects of processing variables and sintering method
2024, Materials Today CommunicationsSolid-state additive manufacturing of dispersion strengthened aluminum with graphene nanoplatelets
2024, Materials Science and Engineering: AInterfacial microstructure evolution and mechanical properties of carbon fiber reinforced Al-matrix composites fabricated by a pressureless infiltration process
2024, Materials Science and Engineering: ASqueeze infiltration processing of lightweight smart aluminum graphite functionally graded composite for enhanced wear resistance
2023, Journal of Manufacturing ProcessesInfluence of carbon fiber failure mode caused by TiO<inf>2</inf> coating on the high temperature tensile strength of carbon fiber reinforced 7075 Al alloy composites
2023, Journal of Materials Research and Technology